The present invention relates to a radio receiver demodulator capable of stably receiving a phase modulated signal. In satellite communication systems, a PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation) system is often employed because of its transmission efficiency. In the case of demodulating a signal that has been modulated by such a modulation system, the carrier is recovered from the modulated signal and used for phase detection. However, when a number of modulation phases is considered as n, carrier frequency as f.sub.o, clock frequency of the received digital signal as f.sub.CL, the recovered carrier is pulled and stabalized at the frequency f.sub.o +m/n f.sub.CL (where m=.+-.1, .+-.2, .+-.3 . . . ) as well as the correct frequency f.sub.o, so the carrier may sometimes be pulled falsely due to noises. Therefore, it is necessary to control the recovered carrier so that it is pulled to the correct frequency and it is expected to form a structure where the carrier is pulled stably to the correct frequency even under the condition of a bad signal to noise ratio.
In order to correct such false pull-in of the recovered carrier, a sweep track type phase lock loop (PLL) circuit is used. In this circuit a phase error signal or a frequency discrimination signal generated by the demodulator being higher than a threshold level indicates an asynchronous condition (i.e., an unlocked condition or a false lock condition). As a result, a sweep signal is applied as a control voltage of a voltage controlled oscillator used in recovering the carrier signal; this pulls the carrier to the correct frequency. The pull-in range is equivalently widened due to sweeping the recovered carrier frequency. The sweep is stopped when a phase lock condition is achieved on the correct frequency.
However, in demodulation utilizing the sweep track type phase lock loop circuit, if a high level noise is included in an input signal, the circuit erroneously detects asynchronization and sweep is started even through the recovered carrier is locked to the optimum phase on the correct frequency. The code error characteristic is therefore deteriorated when the sweep is erroneously started under the phase lock condition on the correct frequency as explained above.
On the other hand, in recent satellite communication systems, the systems are expected to operate normally even when the signal to noise ratio of a received signal is 0 dB or less. In an attempt to achieve this, a highly effective error correcting technique is employed. This error correction technique is expected to be further developed in the future and therefore the demodulator is also expected to operate stably even when a signal to noise ratio is low.